Modification of aluminum-silicon alloys



United States Patent 3,486,884 MODIFICATION OF ALUMINUM-SILICON ALLOYS Michel Baille, Bry-sur-Marne, Val-de-Marne, France, assignor to Foseco International Limited, Nechells, Birmingham, England, a British company No Drawing. Filed Oct. 24, 1966, Ser. No. 588,759 Int. Cl. C22b 9/12; C22c 21/04 U.S. CI. 7568 10 Claims ABSTRACT OF THE DISCLOSURE A method of modifying and maintaining the modification of aluminum-silicon alloys comprises initially modifying the alloy and maintaining modification with a mixture of a sodium salt and a refractory filler. A single addition of a two-part treatment tablet may effect both steps.

This invention relates to the process of modification of eutectic aluminum-silicon alloys and to products for use in the said process. The term modification, with respect to eutectic aluminum-silicon alloys, is applied to the treatment whereby the coarse, needle-like aluminum silicon constituent of the alloy is dispersed in fine form, within the grain structure of the matrix. The treatment also effects an increase in the amount of silicon which can exist in combined form in the aluminum-silicon eutectic. Thus, modification can result in the removal of primary (i.e. uncombined) silicon from the alloy. The effect of modification is an improvement, to a marked degree, in all the mechanical properties of the alloy, particularly as regards ductility and toughness.

It is desirable therefore to produce, in castings made from eutectic aluminum-silicon alloys, a fully modified structure and it is well known to introduce sodium, or a salt thereof, preferably a halide, into the molten alloy for this purpose.

It has, however, been found that the modification to the molten metal caused by the introduction of sodium or a salt thereof, is not permanent, and tends to fade. Generally, after such treatment, the modified state will only persist in the cast product if the time between treatment and casting is less than about minutes. If, as not infrequently occurs, the molten alloy is left to stand, e.g. in a diecasting foundry holding furnace, the melt reverts to the unmodified state, and, in order to achieve the desirable mechanical properties, more sodium or sodium salt must be added. Various factors influence this rate of reversion eg the size of the melt, the silicon content of the alloy and the temperature of the melt. However, even the most favourable selection of conditions does not avoid considerable demodification if the molten metal is allowed to stand for any substantial period.

It is therefore an object of the present invention to provide means for the treatment of aluminum-silicon alloys which promote and maintain in the molten aluminum-slicon alloy, the modified state referred to above.

According to the present invention there is provided a process for the production of eutectic aluminum-silicon alloy castings from a molten said alloy which comprises treating the molten alloy with sodium and/or a sodium salt or salts in a quantity sufficient to modify the alloy to the desired degree of modification in the cast alloy, and maintaining the state of modification so achieved during the interval between such treatment and the casting of the molten metal by the addition of the alloy of "ice a composition comprising as its principal ingredients a sodium salt or a mixture of alkali metal salts including a sodium salt, in admixture with a refractory filler.

If desired, the composition just set forth may be in coherent solid block form and may contain a bonding agent for the ingredients.

It has been found that, in a composition as just described, sodium and/or sodium salt is released into the molten alloy only slowly, so that the tendency to gradual demodification of the alloy on standing is compensated by the introduction into the alloy of fresh sodium and/ or sodium salt.

According to a further feature of the invention there is therefore provided a composition suitable for modifying or maintaining the modified condition of eutectic aluminum-silcon alloys which comprises a sodium salt or a mixture of alkali metal salts including a sodium salt, in admixture with a refractory filler. As noted above said composition may include a bonding agent for the ingredients.

According to a further feature of the invention an additive product for modifying eutectic aluminum-silicon aloys and maintaining the modification thereof comprises a first portion consisting substantially of sodium and/or sodium salt or salts and a second portion comprising a mixture of ingredients consisting substantially of sodium salt or a mixture of alkali metal salts including a sodium salt, refractory filler and optionally a bonding agent bonding said ingredients together. The quantity of the second portion of such a combined product may be adjusted so as to maintain the modified state of the molten alloy for any desired length of time; and if necessary, further quantities of the second portion only may be added to maintain modification.

The form of such an additive product may vary widely. For example, a body of the aforesaid composition may be provided with an outer covering of the sodium and/or sodium salt initially to modify the alloy. For simplicity of manufacture, however, it is generally preferred to provide a multilayer tablet formed from two tablets of bonded or compacted material stuck together and to use the product by floating it on the molten alloy with that portion which consists essentially only of sodium and/ or sodium salt being in contact with the metal. By this means the action of the modification maintaining composition on the molten metal is delayed until the whole or substantially the whole of the sodium and/or sodium salt has entered the molten alloy and thus modified it.

In the formulation of the material used to modify the aluminum initially, as aforesaid sodium metal may be employed. However, it is preferred that in this material a sodium salt or mixture of sodium salts is used. Sodium fluoride is often preferred and for that reason part or even the whole of the sodium salts in the material may be sodium fluoride.

The composition for maintaining the modified condition of the molten alloy will also preferably comprise sodium fluoride, though it may also contain other sodium salts. It should preferably not contain greater than 55% sodium fluoride, to minimise the danger of over-modification.

The refractory material used in the composition may be any of those conveniently used in foundry practice, e.g. sand, or a refractory silicate such as chamotte, or kaolin, or grog. The bonding agent may be, for example, liquid sodium silicate, sulphite lye or an organic gum. The bonding agent may be chosen so that while it holds the ingredients of the composition together it does not wholly prevent the disintegration of the composition at the temperature of the molten alloy with which it is to be used. It may sometimes be advantageous to include in the composition, as part of the refractory filler, a proportion of a material which expands on heating, e.g. perlite or vermiculite, to assist in the disintegration of the composition. In this fashion it is possible to see when the modification maintaining composition is used up, and further tablets (of modification maintaining composition only) may then be added.

For ease of removal of spent composition it may sometimes be preferred to bond the tablet together using a bonding agent such as sodium silicate which will maintain the tablet in block form. It may then be removed with ease after a predetermined period of time.

The preferred proportion of sodium or sodium salt used in the initial modification material will vary with the temperature of the molten alloy, and with the degree of modification required. It is pointed out in this connection that the provision of too great a sodium concentration in the aluminum-silicon alloy melt will impair the mechanical properties of the final castings as compared with the optimum mechanical properties obtainable. The sodium fluoride content of the modifying material is preferably not greater than 55%.

The following examples will serve to illustrate the invention (percentages are by weight of total composition) EXAMPLE I Blocks of additive for the treatment of molten eutectic aluminum-silicon alloys at temperatures of 65()740 C. were made up as follows:

Compositions IA and IB were made up by mixing together the following ingredients:

Composition IA Percent Sodium fluoride 40 Sodium chloride (preferably half fine grade and half coarse grade) 4O Chamotte 20 Composition IB Percent Sodium fluoride 34 Sodium chloride 26 Potassium chloride 40 Composition IA was then pressed into tablets of 750 gm. Wt. each and composition IB into tablets of 500 gm. wt. each. One tablet of each composition was then taken and the two tablets stuck together with a conventional adhesive.

In use, the tablet was placed on top of the molten aluminum-silicon alloy in the ladle, with the composition IA above the composition IB. On contacting the molten metal, composition IB was rapidly consumed by the metal and thus a modified aluminum-silicon alloy of the type which when cast exhibits desirable mechanical properties was produced. The gradual consumption of the block of composition IA, however, prevented the reversion of the molten alloy normally resulting from standing, by maintaining the sodium concentration at the correct level. When the composite tablet was completely used up, further tablets of composition IA only were added at suitable intervals. After standing for several hours the molten metal was poured. This resulted in castings of good mechanical properties possessing a satisfactorily modified structure.

EXAMPLE II Blocks of treatment agent suitable for the treatment of molten eutectic aluminum-silicon alloys at temperatures in excess of 740 C. were prepared as in Example I using the following compositions:

Composition IIA Percent Sodium fluoride 40 Sodium chloride (preferably coarse grade 50 Kaolin 10 Composition IIB was the same as composition IB. The tablets Were formed as before and similar improvements in the cast metal were noted.

EXAMPLE III 250 lb. melt of aluminum-silicon alloy, (type LM6 of. B.S.S. No. 1490), was modified with vacuum melted sodium at 760 C.

A composition consisting of (parts by weight):

Parts Sodium fluoride 30 Sodium chloride 60 Crushed chamotte (-10 mesh) 10 Liquid sodium silicate 5 was made up and formed into blocks of 200 gm. weight. One such block was added to the 250 lb. modified alloy melt, and maintained the modification thereof for about 1.5 hours. At the end of this time the spent block was removed and a fresh one substituted. Castings made from the alloy showed that modification had been satisfactorily maintained.

The foregoing examples serve to indicate methods by which the process of the present invention may be applied to the molten eutectic aluminum-silicon alloys. The compositions of the present invention may also be applied by other methods well known in the art such as in the form of slabs on the walls of the ladle, or in blocks held in the ladle by retaining means such as bars, or other methods.

The size of the blocks may vary widely according to the conditions of use. Generally it will be convenient to use one or more blocks of the size indicated in the examples, of total weight 1.01.5 kg. per block. The recipes may also be varied to suit particular conditions of use, especially with respect to the desired final sodium content and treatment temperature of the molten alloy.

The physical characteristics of the various ingredients may be selected to be most suitable for the subsequent treatment of the compositions. Thus, in composition IA some of the sodium chloride included is preferably of coarse grade to improve the ease with which composition IA may be tabletted, by pressing; and, similarly, the grade of chamotte used in Example III is carefully chosen.

I claim as my invention:

1. A process for the production of eutectic aluminumsilicon alloy castings from a molten said alloy which comprises treating the molten alloy with a first discrete tablet of a sodium-containing material in a quantity sufficient to modify the alloy to the desired degree of modification in the cast alloy, and maintaining the state of modification so achieved during the interval between such treatment and the casting of the molten metal by treating the alloy with a second discrete tablet consisting of a composition consisting of as its principal ingredients a sodum salt and a refractory filler, said first and second tablets being adhered to each other to form a multilayer block.

2. A process according to claim 1 wherein the molten alloy is first treated with vacuum melted sodium.

3. A process according to claim 1 wherein the sodium salt is sodium fluoride.

4. A two part additive product for modifying eutectic aluminum-silicon alloys which consists of a first portion consisting substantially of a sodium-containing material and a second portion attached to said first portion, which second portion is a mixture of ingredients consisting essentially of a sodium salt and a refractory filler, said first portion encasing said second portion.

5. A two part additive product for modifying eutectic aluminum-silicon alloys which consists of a multilayer block consisting of a first portion consisting substantially of a sodium-containing material and a second portion at tached to said first portion, which second portion is a mixture of ingredients consisting essentially of a sodium salt and a refractory filler, said first and second portions being discrete tablets.

6. The product of claim 5 wherein said sodium salt is selected from the group consisting of sodium chloride and sodium fluoride and the refractory filler is selected from the group consisting of chamotte and kaolin, with the proviso that when the sodium salt is sodium fluoride it constitutes not more than 55% by weight of the composition.

7. An additive product according to claim 5 wherein the first portion consists essentially of a salt selected from the class consisting of sodium chloride and sodium fluoride.

8. An additive product according to claim 5 wherein the second portion includes a bonding agent.

9. An additive product according to claim 8 wherein the refractory filler is a member selected from the group consisting of chamotte and kaolin.

10. An additive product accoring to claim 8 wherein the bonding agent is aqueous sodium silicate.

References Cited UNITED STATES PATENTS 1,794,401 3/ 1931 Haglund 7594 X 1,797,131 3/1931 DeBoer 7566 1,848,798 3/1932 Pacz 7568 2,013,926 9/1935 Pacz 7593 X 2,497,745 2/1950 Stohr 7594 X 2,654,670 10/1953 Davis et a1. 7568 2,823,989 2/1958 Deyrup et al. 75130 X 3,144,323 8/1964 Watson et al 75-68 X 3,151,980 10/ 1964 Anderko et a1. 7593 X FOREIGN PATENTS 931,755 7/ 1963 Great Britain. 990,125 4/ 1965 Great Britain.

L. DEWAYNE RUTLEDGE, Primary Examiner HENRY W. TAR'RING, II, Assistant Examiner US. Cl. X.R. 

